Originally Posted by
crawdaddio
Blackburn flea.
http://www.blackburndesign.com/lights.html
Mounts with a velcro strap. Easy on and off, super duper lightweight. Can be switched between bikes in about 10 seconds. Rechargeable with ANY 1.5V battery. Waterproof. Lifetime warranty. Very bright. Can be found for about $20 a piece. Highly recommended.
PB superflash is good too.
I just got one and its blindingly bright! The size is key too. Its the smallest I've found. All the blackburn rear lights are pretty nice actually. The flea comes by itself or in a combo front/rear set.